It is known in the art to use polymer dispersions as sizing agents in the the production of cellulose based products such as paper, board and paper board. The sizing agents delay or prevent the absorption and spreading of aqueous solutions in the sized products. A wide variety polymer dispersions have been disclosed in the prior art including those prepared by emulsion polymerization of ethylenically unsaturated monomers in the presence of emulsifiers and/or protective colloids which impart stability to the dispersions so formed. The dispersions can be used for surface sizing which involves applying the dispersion to the surface of the product to be sized. Examples of monomers used for this purpose include styrene and alkyl (meth)acrylates. Such surface sizes are among the most efficient now in use and usually provide high sizing response.
Besides having the capability of producing cellulose based products with the desired degree of resistance to penetration by aqueous liquids, such as for example printing and writing inks, the polymer dispersion to be used for surface sizing should be beneficial also from an application point of view. The surface size is usually applied to the surface of the cellulose based product in the size press. In order to prevent static electricity being built up during the application or converting process, it is common to add electrolytes into the size press. However, it is known that even low levels of electrolytes may cause the polymer particles contained in aqueous size dispersions to agglomerate and form deposits, leading to application problems such as difficulty in dosing the size dispersion, deteriorated size performance and poorly sized paper. In order to achieve optimum results in surface sizing it is thus desired that the polymer dispersions have good stability in the presence of bases and electrolytes. It is furthermore beneficial to the application process that the polymer dispersion has a low tendency to foaming and advantageous viscosity and flowability profiles.
It is, accordingly, an object of the present invention to provide an improved method of surface sizing cellulose based products. Another object of the invention to provide an aqueous dispersion containing emulsion polymerized monomers which results in improved surface sizing. It is another object of the invention to provide an aqueous dispersion containing emulsion polymerized monomers which has beneficial application characteristics in surface sizing and, in particular, high performance in the presence of electrolytes and bases. Other objects of the invention will become apparent.